1. Field of the Invention
The present invention relates to an apparatus for measuring the partial gas pressure in liquid media.
2. Description of the Related Art
Primarily in the field of fermentation technology, it has increasingly been found necessary to carry out the measurement of gases through the determination of the partial pressure. For example, special probes have been developed for the determination of the partial oxygen pressure and the partial carbon dioxide pressure. A widely used example of these probes are the so-called Severinghaus electrodes. These devices operate with diaphragm-covered single-rod pH-electrodes. (DE-OS 25 08 637, Biotechnol. Bioeng. 22(1980), 2411-2416, Biotechnol. Bioeng. 23(1981), 461-466). In this system, an electrolyte solution or an electrolyte paste is located between the gas-selective diaphragm and the pH-electrode. The measuring principle is based on the fact that in an aqueous solution carbon dioxide forms carbonic acid which dissociates into a bicarbonate anion and a proton. This process results in the electrolyte solution in a pH-value change which is measured by means of the pH-probe. The disadvantage of this measuring principle is the fact that carbon dioxide is not measured directly, but in its ionic form. Since the portion of the ionic form is below 0.1%, this method is not sufficiently accurate. In addition, other volatile acid or basic gases impair the pH-value measurement. Also, the maintenance costs are very high.
Also known from the prior art are pCO.sub.2 -optodes. These optodes also include a diaphragm-covered sensor system (SPIE Vol.798 Fiber Optic Sensors II (1987) p. 249/252; Anal. Chim. Acta 160 (1984) p. 305-309; Proc. Int. Meeting on Chemical Sensors, Fukuoka, Japan, Elsevier, p. 609-619, (1983), Talanta 35(1988)2 p. 109-112. Anal. Chem. 65(1993) p.331-337, Fresenius Z. Anal. Chem. 325(1986) p.387-392). The pH-optodes use as indicator phase pH-indicators which change their absorption properties or fluorescence properties in dependence on the proton concentration (Anal. Chem. 52(1980) S. 864-869, DE-OS 3 343 636 and 3 343 637. U.S. Pat. No. 855,364). When the indicator is separated from the substance being measured by means of a gas-permeable diaphragm, only gases, for example, carbon dioxide, can pass through the diaphragm to the indicator phase and can produce in the indicator phase a pH-value change by hydrolysis. The operation of such carbon dioxide optodes is analogous to the Severinghaus electrodes. The disadvantages of optical pH-measurements and, thus, pCO.sub.2 -measurements, are the very limited analytic measuring range and the dependency on ionic strength. The disadvantages already mentioned in connection with the Severinghaus electrodes also prevent the wide use of the optodes.
A differential pressure gage for the determination of carbonic acid is known from German Offenlegungsschrift 2 435 493. However, this device can only be used in flowing media. Accordingly, the device is particularly not suitable for use in conventional stirring or stationary reactors, as they are used particularly in the fermentation industry.
German Offenlegungsschrift 2 926 138 discloses a device for the continuous measurement for the content of dissolved carbon dioxide in liquids. The measuring principle is based on the determination of the conductivity differential. The device is equipped with a diaphragm, wherein liquid containing dissolved carbon dioxide flows against one side of the diaphragm and a neutral or basic measuring liquid flows against the other side of the diaphragm. A conductivity transducer each is arranged in the flow path of the measuring liquid in front of and following the permeable diaphragm. This measurement has the disadvantage that it is not suitable for liquids which change their chemical and physical properties.
In addition, it is known from European patent application 0 462 755 to determine gases, such as CO.sub.2, by infrared absorption measurement. In this measurement, a ray of infrared light is conducted through the fluid to be measured. The light ray is divided into two or more components. These divided light rays are then measured. This measuring arrangement has the disadvantage that is does not permit the measurement of partial pressures and that it is sensitive to dispersive particles of the sample liquid.
The division into two ray paths is already known from GB 2 194 333. In this method, only one light ray is conducted through the substance to be measured. The remaining radiation is used as reference light in order to increase the accuracy.
Also known in the art is a so-called chopped gas analysis device which also operates with luminescence diodes (Laser und Optoelektronik 17(1985)3, p. 308-310, Wiegleb, G.: Einsatz von LED-Strahlungsquellen in Analysengeraten [Use of LED-radiation sources in analysis devices]).
The known devices and methods have in common that they only measure concentrations. The substance to be measured is placed directly and measured in the path of radiation. This is possible for gases and liquids without dispersive particles with media-constant compositions in which disturbances can be taken into consideration by a blank value. However, the above-described optical methods are not capable of determining partial pressures. Also not possible is a use for liquids having compositions which change with the media and which contain particles which render the liquid turbid.
U.S. Pat. No. 5,116,759 discloses a measuring probe in which a measuring volume is defined by a diaphragm. Substances to be detected can diffuse through the diaphragm into the measuring volume where they react chemically with a sensing reagent. Thus, for example, a CO.sub.2 measurement is based on a change of the pH value. The change of the reagent is then determined by measuring. This system has the disadvantage that the chemical sensing reagent is required. The sensing reagent and the diffusion process lead to a relatively long response time to the changes of the partial pressure in the sample space, wherein the response time may be in the order of magnitude of seconds to minutes because of the required diffusion into the measuring chamber. Another problem is the use in the case of very high partial pressures, because in that case too much light is absorbed and the measuring signal is too weak as a result.
Also known and described in the art is the analytic method of attenuated total reflectants (ATR). The measurement utilizes the phenomenon of the formation of evanescent waves or surface waves at the border surface between two optically differently dense media. In a medium having a high refraction number, a light ray is reflected back into the optically denser medium at the border surface to an optically thinner medium if the angle between the incident light ray and the perpendicular to the border surface exceeds the border angle of the total reflection. However, a portion of the light rays penetrate into the surrounding thinner medium by a few wave lengths and are only there reflected back into the optically denser medium. If light-absorbing substances are present in the area of this short light path, the reflected portion of the light becomes smaller. This weakening can be detected and correlated to the quantity of the absorbed substance.
A number of embodiments of the method for utilizing this light absorption phenomena are known in the art. Most ATR devices contain crystals, usually trapezoidally cut prisms. In DE-OS 42 27 813, very simple geometric shapes are described for the ATR element. Commercially available plane-convex microlenses of glass or of synthetic material, having the shape of hemispheres, are used.
In DE-OS 44 18 180, a cube corner reflector in the form of a triple prism is used. The advantage of this arrangement is its compact construction. The emitted light is deflected by 18.degree.. This makes it possible to use the arrangement in a thin rod. Light wave conductors are used for supplying the light to be introduced and for removing the residual light.
DE-OS 40 38 354 discloses an ATR probe which does not use any prisms, lenses or similar components. The light is also conveyed through light conductors. The light conductors for supplying and removing light and the actual ATR sensor are composed of a common light conducting fiber, wherein the casing of the light conductor is removed in the area of the probe which does participate in the reaction. The light wave conductor is mechanically supported and is arranged in a probe body within a measuring chamber, so that the light wave conductor is in contact with the medium to be examined.
It is also known in the art to determine the CO.sub.2 concentration in liquids by means of weakened total reflection (The Chemical Engineer 498 (1991) page 18). In a flow measuring cell for fluid substances, for example, beer, an attenuated total reflectance crystal (sapphire-ART) is arranged perpendicularly of the flow direction. The infrared light which is supplied at one side to the crystal travels through the crystal and is totally reflected several times. Each reflection causes the radiation to penetrate into the sample liquid by several .mu.m and is weakened by the carbondioxide present in the substance The residual light liquids or gases can be used in the same manner. The type of the fluids depends on the gases to be measured.
In accordance with another preferred feature of the present invention, luminescent diodes are used as the light emission source. The use of these devices has the following advantages:
The emission has a relatively narrow band width, i.e., the use of interference filters is not absolutely necessary for selectively determining the respective gas. Because of the relatively low current consumption, it is possible in principle to construct the measuring arrangement with battery operation. A decisive advantage as compared to conventional infrared sources is the high constancy of power. Therefore, it may be possible to omit a reference circuit or to set up compensation circuits without moving parts. Such a system is less susceptible to mechanical trouble. At the same time, the high constancy of power ensures a long operation without recalibration. The luminescent diodes have such a small dimension that coupling of the light into optical waveguides is possible without problems. Thus, the sensitive components can be positioned externally and are not subject to the thermal and mechanical loads of a vapor sterilization.